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| United States Patent |
5,624,706
|
|
Goukassian
|
April 29, 1997
|
Method for fabricating electron multipliers
Abstract
A method for fabricating an electron multiplier is provided. The method
consists of depositing a random channel layer on a substrate such that the
random channel layer is capable of producing a cascade secondary electron
emission in response to an incident electron in the presence of an
electric field.
| Inventors:
|
Goukassian; Samuel (Livonia, MI)
|
| Assignee:
|
Electron R+D International, Inc. (Farmington, MI)
|
| Appl. No.:
|
440754 |
| Filed:
|
May 15, 1995 |
| Current U.S. Class: |
427/77; 427/78; 427/126.1; 427/231; 427/237; 427/238; 427/255.5; 427/294 |
| Intern'l Class: |
B05D 005/12 |
| Field of Search: |
427/77,78,126.1,231,237,238,255.5,294
|
References Cited
U.S. Patent Documents
| 3739216 | Jun., 1973 | Pakswer | 427/78.
|
| 3979621 | Sep., 1976 | Yates | 313/105.
|
| 3979637 | Sep., 1976 | Siegmund | 315/12.
|
| 4005323 | Jan., 1977 | Yates et al. | 313/105.
|
| 4010019 | Mar., 1977 | Cole et al. | 65/36.
|
| 4088510 | May., 1978 | Dresner et al. | 148/6.
|
| 4099079 | Jul., 1978 | Knapp | 313/103.
|
| 4153855 | May., 1979 | Feingold | 96/44.
|
| 4365150 | Dec., 1982 | Bateman | 250/207.
|
| 4395437 | Jul., 1983 | Knapp | 427/78.
|
| 4639638 | Jan., 1987 | Purcell et al. | 427/77.
|
| 4777403 | Oct., 1988 | Stephenson | 313/533.
|
| 4945286 | Jul., 1990 | Phillips et al. | 313/105.
|
| 4976988 | Dec., 1990 | Honda | 427/42.
|
| 4980604 | Dec., 1990 | L'Hermite | 313/533.
|
| 5086248 | Feb., 1992 | Horton et al. | 313/103.
|
| Foreign Patent Documents |
| 795206 | Apr., 1979 | SU.
| |
| 824808 | Dec., 1979 | SU.
| |
| 1023446 | May., 1981 | SU.
| |
Other References
Wiza, Joseph Ladislas. "Microchannel Plate Detectors," Nuclear Instruments
and Methods 162 (1979) 587-601.
|
Primary Examiner: Utech; Benjamin
Attorney, Agent or Firm: Brooks & Kushman P.C.
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a continuation-in-part of U.S. application Ser. No.
321,042, filed Oct. 5, 1994, now abandoned which is a continuation of U.S.
patent application Ser. No. 092,083, now abandoned filed Jul. 15, 1993,
both of which are hereby incorporated by reference.
Claims
What is claimed is:
1. A process for the preparation of a micro random channel plate,
comprising:
a) providing a substrate;
b) providing a deposition substance or precursor thereof, said deposition
substance having a coefficient of secondary electron emission greater than
1;
c) depositing said deposition substance onto said substrate forming a
performing substance layer comprising a plurality of chaotically situated
dynodes interspersed with open space constituting random microchannels;
wherein said performing substance layer generates secondary electron
emission in the presence of an electric field in response to an incident
electron.
2. The process of claim 1 wherein the apparent density of said performing
substance layer is from 0.3% to 4% of the density of the substance
deposited when in the monocrystalline state.
3. The process of claim 2 wherein said performing substance layer has a
thickness of from 1 .mu.m to 3000 .mu.m.
4. The process of claim 1 wherein the apparent density of said performing
substance layer is from 0.5% to 2% of the density of the substance
deposited when in the monocrystalline state.
5. The process of claim 4 wherein said depositing takes place at a pressure
of between 1 and 10 torr.
6. The process of claim 5 wherein said performing substance layer has a
thickness of from 1 .mu.m to 3000 .mu.m.
7. The process of claim 1 wherein said deposition substance is deposited on
said substrate from the vapor state.
8. The process of claim 1 wherein said depositing takes place at a pressure
of between 1 and 10 torr.
9. The process of claim 1 wherein said deposition substance is selected
from the group consisting of the halides, oxides, and sulfides of the
metals of Groups Ia, IIa, and IIIa of the Periodic Table, or mixtures
thereof.
10. The process of claim 1 wherein said performing substance layer has a
thickness of from 1 .mu.m to 3000 .mu.m.
11. The process of claim 1 wherein said dynodes comprise crystals of said
deposition substance.
12. The process of claim 1 wherein said dynodes comprise crystallites of
said deposition substance.
13. The process of claim 1, wherein said substrate comprises a microchannel
plate device.
14. A process for the preparation of a micro random channel plate device
having a performing substance layer comprising a plurality of chaotically
situated dynodes having interspersed therebetween a plurality of random
microchannels, said micro random channel plate device generating secondary
electron emission in the presence of an electric field in response to an
incident electron, said process comprising:
a) providing a substrate;
b) providing a deposition substance or precursor thereof, said deposition
substance having a coefficient of secondary electron emission greater than
1;
c) heating said deposition substance to a temperature below its melt
temperature at a first pressure less than atmospheric to dry said
deposition substance;
d) increasing the temperature of said deposition substance to a temperature
at least equal to its melt temperature at a second pressure in the range
of 1 to 10 torr and melting said deposition substance; and
e) positioning said substrate relative to said molten deposition substance
such that said deposition substance condenses onto said substrate to form
said performing substance layer.
15. The process of claim 14 wherein said deposition substance is enclosed
in a fencing cylinder, and said substrate during deposition is positioned
at an open end of said fencing cylinder.
16. The process of claim 15 wherein the distance between said molten
deposition substance and said substrate is between 40 mm and 100 mm.
17. The process of claim 15 wherein said open end of said fencing cylinder
is closed by means of a removable screen, and following the melting of
said deposition substance said screen is removed and said substrate
positioned at said open end of said fencing cylinder.
18. The process of claim 14 wherein said performing substance layer has a
depth of from 1 .mu.m to 3000 .mu.m, and wherein the density of said
performing substance layer is from 0.3% to 4% of the density of the
substance deposited when in the monocrystalline state.
19. The process of claim 14 wherein said substrate is a microchannel plate
device.
Description
TECHNICAL FIELD
The present invention relates to methods for fabricating electron
multiplier devices, and to electron multiplier devices prepared thereby.
BACKGROUND ART
Microchannel plate electron multipliers (MCPs) are continuous dynodes
generally consisting of glass with a high lead oxide content, made
slightly conductive through a hydrogen firing process. The glass tubes
forming the microchannels have electrodes at the entrance and exit. The
entrance can be conical or straight, while the main section is straight,
bent, or spiraled. The output current has to be a tenth or less of the
strip current, otherwise the multiplier operates in a saturated mode. The
amplification depends on the length-to-diameter ratio of the multiplier,
the axial field strength, and the secondary-electron-emitter material.
Fabrication of microchannel plate electron multipliers and their
characteristics is described by J. L. Wiza, "Microchannel Plate
Detectors", NUCLEAR INSTRUMENTS AND METHODS, 162 (1979), pp. 587-601.
Glass rods having a core of etchable glass and cladding of non-etchable
lead glass are stacked into hexagonal arrays and drawn down to smaller
size. The resulting arrays are again stacked into hexagonal arrays and
redrawn to finer size, following which they are stacked and fused within a
glass envelope to form a boule. The boule may be sliced orthogonally or at
an angle to the boule axis, the surface of the resulting plates polished,
and the core glass etched away, leaving an array of closely spaced,
hollow, cylindrical microchannels in a regular geometric pattern. The
plates are subjected to hydrogen reduction at elevated temperature,
forming a layer of semiconductive lead on the tube interior surfaces. The
front and back of the plates are then metallized to form conductive
electrode surfaces.
The microchannel plates thus produced may be used singly or stacked to form
multiple layer arrays. The cost of such devices is high, due to the
multiple drawing and other steps required, and the useful gain limited by
ion feedback and dark noise. Ion feedback is greatly increased at
operating pressures above 10.sup.-6 torr, and thus operation generally
requires a relatively high vacuum, 10.sup.-5 torr, and preferably
10.sup.-6 torr or below. Size is limited both by the nature of the
manufacturing process as well as the necessity of providing a supporting
envelope capable of withstanding the requisite internal/external pressure
differential.
Thus, although such MCPs exhibit numerous advantages, they also exhibit the
following problems:
a) They utilize highly complicated technology;
b) They are relatively costly to produce;
c) The sensor area is limited to approximately 125 mm.sup.2 ;
d) Their effective surface (permeability or open area) is typically on the
order of only 50%-60%;
e) They have a relatively high dark noise (0.5 s.sup.-1 cm.sup.-2);
f) They can only be economically produced in a flat configuration;
g) They are made of material restricted to one type;
h) They function only in high vacuum.
All of the above constraints prevent MCPs from being used more widely,
particularly in areas of medical and industrial diagnostics, flat screens,
nuclear science and others.
Improvements in microchannel plate devices have centered on making them
more economical to produce, or maximizing their operating parameters, for
example by increasing collection efficiency, decreasing ion feedback, or
increasing secondary electron emission, thus increasing gain. However,
little attention has been devoted to altering the basic means of
manufacture.
In Knapp, U.S. Pat. No. 4,395,437, for example, dynodes are prepared from
mild steel perforated with numerous holes in a regular geometric array to
form microchannels rather than use multiple glass drawings to prepare the
microchannel plate. Layers of magnesium and aluminum are then formed on
the dynode by evaporation of the metals at a pressure of 1 to
3.times.10.sup.-5 torr. The coating is then allowed to oxidize in ambient
air and activated by heating for several hours in oxygen at pressure in
the range of 10.sup.-4 to 10.sup.-5 torr. The micro-channel plates are
then stacked with insulating spacers to form a multiple layer,
microchannel device. No indication of gain is given, but secondary
electron emissions on the order of 8 are achieved. A similar concept,
using a metal/ceramic (cermet) coating is disclosed by Knapp in U.S. Pat.
No. 4,099,079.
Pakswer, in U.S. Pat. No. 3,739,216, discloses both conventional high
vacuum electrostatic electron multiplier tubes as well as microchannel
multiplier plates where improved secondary emission at low cross-over
voltage is achieved, as in Knapp '079, by the use of a cermet coating
consisting of metal globules 40-500 .ANG. in size dispersed in a ceramic
matrix, forming a single layer thin film having a thickness of from
200-2000 .ANG. (0.02-0.2 .mu.m). In microchannel plate devices, this film
may be vacuum-sputtered or deposited by chemical vapor deposition into the
tubular glass microchannels of devices such as those disclosed by Wiza.
The devices suffer from the same drawbacks as those of Wiza, requiring
high vacuum for operation. The metal globules lower the resistivity of the
matrix.
It would be desirable to provide an electron multiplying device which is
capable of more cost-effective manufacturing than microchannel plate
devices currently available. It would further be desirable to provide a
microchannel plate electron multiplier capable of being produced in
greater variety of sizes and shapes than those presently available. It
would be yet further desirable to provide a microchannel electron
multiplier having high gain and low dark noise, and to provide a method
for the production of such devices.
The micro random channel plate devices of the subject invention have solved
many of the problems inherent in microchannel plate electron multipliers
of the prior art. The subject devices provide a structure comprising
randomly situated crystals, the interstices between which form randomly
oriented channels for producing secondary emissions. These devices may be
efficiently and inexpensively fabricated on not only flat surfaces but on
surfaces of other shapes, and importantly, surfaces of widely varying size
as well.
SUMMARY OF THE INVENTION
It is thus an object of the present invention to provide a method for
fabricating electron multipliers which is both efficient and inexpensive.
Further, an object of the present invention is to provide a method for
fabricating electron multipliers of relatively large size as a result of
simple technology.
Moreover, an object of the present invention is to provide a method for
fabricating electron multipliers such that the actuating surface is nearly
100% of the actual surface.
An additional object of the present invention is to provide a method of
fabricating electron multipliers which result in a low dark noise.
An object of the present invention is also to provide a method for
fabricating electron multipliers which allows the formation of a secondary
electron emitting plate whose surface is of any arbitrary configuration.
An object of the present invention is further to provide a method for
fabricating an electron multiplier such that a variety of different
deposition substances may be used to achieve the necessary performance
characteristics.
It is an additional object of the present invention to provide a method for
fabricating an electron multiplier which is capable of forming channels
which prevent the through traffic of ions.
In carrying out the above objects, the present invention provides a method
for fabricating an electron multiplier. The method comprises the steps of
providing a substrate, and depositing on the substrate a random channel
layer of deposition substance capable of generating secondary electron
emission in the presence of an electric field in response to an incident
electron.
In further carrying out the above objects, the present invention provides a
method for fabricating a multiplying element for an electron multiplier.
The method comprises the steps of providing a pressure controllable
chamber, generating a vacuum within the pressure chamber, and heating a
deposition substance to a temperature below the melting point of the
substance so as to dry the deposition substance. Further, the pressure in
the pressure chamber is increased, and the temperature of the deposition
substance is increased so as to create a transition of at least a portion
of the substance to a vapor. A substrate is positioned in proximity to the
deposition substance so that the deposition substance vapor is condensed
upon the substrate wherein the deposition substance forms a low density
random channel layer on the substrate, the layer capable of generating a
secondary electron emission in the presence of an electric field in
response to an incident electron.
The objects, features and advantages of the present invention are readily
apparent from the following detailed description of the best mode for
carrying out the invention when taken in connection with the accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 represents a schematic diagram of an apparatus used for implementing
one embodiment of the present invention;
FIG. 2 presents a flow chart representation of one embodiment of the
present invention;
FIG. 3 presents a flow chart representation of an alternative embodiment of
the present invention;
FIG. 4 presents a schematic diagram of an electron multiplier of one
embodiment of the present invention;
FIG. 5 presents a schematic diagram of an exemplary embodiment of an
electron multiplier of the present invention; and
FIG. 6 presents a schematic diagram of an exemplary embodiment of an
electron multiplier of the present invention.
BEST MODE FOR CARRYING OUT THE INVENTION
The present invention relates to micro random channel plate (MRCP) electron
multipliers (EM) and their fabrication. An MRCP EM consists of two
electrodes with certain distance in between, between which is a plate of
performing substance, hereinafter a micro random channel plate. The micro
random channel plate of the MRCP EM is fabricated by particular deposition
of substances with high coefficient of Secondary Electron Emission (SEE),
as more fully described hereinafter.
The structure of the plate depends upon many parameters of the deposition
process. The main parameters are: The environmental content, temperature
and pressure where the product is deposited; The quality, purity and
temperature of product that is being deposited; the intensity of
deposition; and the distance from depositor's surface to the substrate. By
adjusting these parameters in selected ways, the micro random channel
plate will have many channels of a particular size and with particular
characteristics.
The micro random channel plates of the subject invention, unlike the
microchannel plate devices of the prior art, contain randomly situated
channels consisting of open space between chaotically situated dynodes
which comprise crystals or aggregates of crystals of a substance having a
secondary electron emission yield greater than one. While in prior art
devices the microchannels are purposefully created with a given diameter
and depth and positioned in a predetermined regular geometric array, in
the subject devices the position and location of the channels is not
predetermined nor is the diameter or depth of the channels fixed. Rather,
by the process utilized to form the subject micro random channel plates,
the crystal growth of the secondary electron-emitting substance is itself
random. The result is a micro random channel plate containing a performing
substance layer consisting of multitudinous discrete dynodes arranged
chaotically. Between the chaotically arranged dynodes are microchannels
defined by the distance between adjacent dynodes. As the dynodes are
chaotically located, the channels by their nature are random in location,
diameter, and length.
Thus, the method of production of the micro random channel plates of the
subject invention is far different from conventional coating techniques
such as RF sputtering, vacuum deposition, vapor deposition, or
conventional chemical vapor deposition. In each of these traditional
coating techniques, the aim is to form a uniform coating, generally one
having a particular, predetermined crystalline orientation, generally a
hard, smooth, thin coating. In order to do so, such coating methods are
performed in relatively high vacuum, generally with the deposition source
proximate to the surface to be coated. The coatings thus produced have
substantially the same density as the deposition substance, with the
exception of chemical vapor deposition where, due to the decomposition of
the chemical precursor substance into a compound of simpler stoichiometry,
the density of the coating is generally higher than the density of the
decomposition substance. For example, a volatile titanium organosulfide
may be chemically vapor-deposited to form a titanium disulfide coating.
In contrast, preparation of the micro random channel plate devices of the
subject invention involves deposition under conditions where a smooth,
high density coating is not formed. Rather, the conditions are
purposefully selected such that crystal growth is random rather than
consistently oriented, and of low rather than high density. One method of
producing this random, or "chaotic" crystal growth, is to maintain the
pressure in the deposition chamber at a much higher pressure than utilized
for other coating methods, for example 1 to 10 torr rather than 10.sup.-2
to 10.sup.-9 torr, and to maintain a suitable distance of separation
between the deposition substance and the substrate. Rotation of the
substrate on which the deposit forms aids in both randomizing crystal
growth and orientation as well as generating a uniform surface, uniform
being used in the sense of having the same thickness, density, and degree
of randomness in crystalline orientation.
Moreover, and again in contrast to previous coating techniques, the
apparent density of the deposited performing substance layer is far less
than the density of the substance deposited, for example from 0.3 about 4
percent of the deposition substances density, preferably 0.5 to about 2
percent. For example, with cesium iodide as the secondary electron
emission substance, the apparent density of the cesium iodide on the micro
random channel plate device will be only about 0.6% of the density of
solid cesium iodide. Thus, the coating may be likened to a coating of dry
snow as compared to a solid film of ice, the snow crystals being
chaotically positioned with respect to each other, and separated by
distances which provide for numerous, randomly oriented channels between
them.
Thus, the subject invention provides a process for the preparation of a
micro random channel plate which includes providing a substrate, and
providing a deposition substance or precursor thereof, the deposition
substance having a coefficient of secondary electron emission greater than
1. The deposition substance is deposited onto the substrate to form a
performing substance layer comprising a plurality of chaotically situated
dynodes interspersed with open space constituting random microchannels.
The location of the dynodes and microchannels is non-predetermined. The
performing substance layer generates secondary electron emission in the
presence of an electric field in response to an incident electron.
The subject invention further provides a process for the preparation of a
micro random channel plate device having a performing substance layer
including a plurality of chaotically situated dynodes having interspersed
therebetween a plurality of random microchannels. The micro random channel
plate device generates a secondary electron emission in the presence of an
electric field in response to an incident electron. The process includes
providing a substrate and providing a deposition substance, or precursor
thereof, which has a coefficient of secondary electron emission greater
than 1. The deposition substance is heated to a temperature below its melt
temperature at a first pressure less than atmospheric to dry the
deposition substance, following which the temperature of the deposition
substance is increased to a temperature at least equal to its melt
temperature at a second pressure in the range of 1 to 10 torr, which melts
the deposition substance. The substrate is positioned relative to the
molten deposition substance such that the deposition substance condenses
onto the substrate to form the performing substance layer.
In order to produce an electron multiplying effect, the average number of
secondary electrons emitted must exceed the number of incident electrons.
Virtually any secondary electron emissive substance meeting this
requirement may be utilized, provided that it may be deposited in the form
hereinbefore described, i.e., as chaotically oriented dynodes having
between them random microchannels of the proper dimensions. Examples of
suitable substances, for example, are alkali halides such as the
fluorides, chlorides, bromides, and iodides of lithium, sodium, potassium,
rubidum, or cesium; the alkaline earth metal halides such as the
fluorides, chlorides, bromides or iodides of beryllium, magnesium, barium,
or strontium; metal oxides such as those of aluminum, magnesium,
beryllium, calcium, barium, and silicon; sulfides such as lead sulfide and
zinc sulfide, and the like. Also suitable are sodium aluminum fluoride,
and other bimetallic halides and oxides. Metals may also be used, however
the secondary electron emission yield is less than with the substances
heretofore mentioned. Those familiar with the secondary electron emission
properties of substances can readily augment this list.
The MRCPs of the subject invention can be fabricated on practically any
surface. The surface may be of almost unlimited size and need not be flat.
The fabrication process is inexpensive and adaptable to various
configurations such as thin wire, tiny matrix, etc. known to those
familiar with micro channel plate electron multipliers. The MRCP electron
multiplier performs in low as well as high vacuum with performing
parameters equal to, or better than those of micro channel plate devices.
Turning now to FIG. 1, a flow chart representation of one embodiment of the
method for fabricating the electron multiplier plate of the present
invention is presented. The method begins by providing a substrate as
shown in block 30. A random channel layer of performing substance is
deposited on the substrate as shown in block 32. This performing substance
is capable of generating a cascade secondary electron emission in the
presence of an electric field in response to an incident electron. A
crystalline layer may be formed from the condensation of the deposition
substance in a vaporous state such that the crystals are oriented randomly
with respect to the substrate. Similarly, other methods of deposition may
be used such as chemical processes capable of forming the random channel
structure.
The substrate mentioned above may consist of an anode or a cathode used for
supplying an electric potential to the plate. Alternatively, the substrate
may provide a support substance for the deposition process which is in
turn removed from the performing substance by some manner, such as
etching, after the deposition has been performed.
FIG. 2 presents a flow chart representation of an alternative method for
fabricating an electron multiplier of the present invention. The method
begins by providing a pressure chamber as shown in block 40. A vacuum is
generated within the pressure chamber as shown in block 42. A deposition
substance within the chamber is heated to a temperature below the melting
point of the substance so as to dry the deposition substance as shown in
block 44. The pressure within the pressure chamber is then increased as
shown in block 46. The ambient temperature inside the cylinder enclosing
the deposition substance is adjusted as shown in Block 47. The temperature
of the deposition substance is increased so as to create a transition of
at least a portion of the deposition substance to a vapor as shown in
block 48. A substrate is positioned over the cylinder in proximity to the
deposition substance so that deposition substance vapor is condensed upon
the substrate as shown in block 50. The plate is moved so as to promote an
even distribution of condensed deposition material as shown in block 52,
wherein the deposition substance forms a random channel layer on the
substrate with the crystals of the porous layer oriented randomly with
respect to the substrate, and wherein the layer is capable of generating a
secondary electron emission in the presence of an electric field in
response to an incident electron.
In the preferred embodiment, the substrate is rotated approximately 200
times during the deposition of the crystalline layer. Thus, for a
deposition time of ten minutes, the substrate is rotated at an angular
velocity of 20 revolutions per minute, and for a deposition time of one
minute, the substrate is rotated at an angular velocity of 200 revolutions
per minute. The rate of rotation may easily be adjusted to maximize
performance parameters.
FIG. 3 presents a schematic representation of an apparatus for making the
EM plate of MRCP. Situated in a bell jar or similar pressure chamber 10
are: manipulator 12, electric motor 14, screen 16, substrate 18, random
channel plate 19, resistive evaporator 20, deposition substance 22,
fencing cylinder 24 and temperature regulator 26.
The fabrication process starts by creating a vacuum of approximately
10.sup.-2 torr under the bell jar 10. The top end of the cylinder 24 is
closed by the screen (shutter) 16. The evaporator 20 is turned on and its
temperature is kept below the melting point of the deposition substance 22
until it is dry. Then the pressure under the jar is brought up to 1-10
torr, determined experimentally depending on the particular deposition
substance 22 being used. A particular pressure is suitable if the process
yields an electron multiplier with a suitable gain. Temperature of the
evaporator 20 is raised slightly above the melting point of the substance
22. Simultaneously the necessary ambient temperature is created in the
cylinder 24 by temperature regulator 26, to create micro crystals of the
necessary size. At that point the screen 16 is moved aside, the substrate
is situated above the cylinder 24. The motor 14 is turned on, and the
substrate 18 is rotated to create even plate 19 of performing substance 22
on the substrate.
After the plate 19 has reached the necessary thickness, the substrate 18
and plate 19 are moved aside and the plate is ready. The pressure under
the bell jar is brought up to ambient, the electron multiplier plate is
removed from the bell jar. As previously mentioned, the substrate 18 may
be an anode or cathode used for supplying an electric potential to the
plate. The second electrode is situated over the plate in a manner that it
is barely touching the plate, or in other orientations as will be apparent
to those skilled in the art of electron multipliers.
The thickness, d, of the dynode layer must be such that it is greater than
L, the average electron track length, and suitable thicknesses may range
from 1 .mu.m to 3000 .mu.m, preferably from 200 .mu.m to 600 .mu.m. The
thickness is adjusted by varying the rate and time of deposition,
parameters which can be determined by routine experimentation. The
apparent density of the performing substance may be calculated from the
volume of the performing substance (thickness multiplied by area of
performing substance) and the weight of the performing substance. As
indicated previously, this apparent density should be in the range of 0.3
to 4 percent of the density of the performing substance when in the
monocrystalline state. For example, monocrystalline cesium iodide has a
density of 4.5 g/cm.sup.3. When deposited in accordance with the subject
invention to form a dynode layer, the apparent density of the cesium
iodide dynode layer should range from c.a. 0.013 to about 0.18 g/cm.sup.3.
This layer is formed of crystals and crystallites optionally together with
non-crystalline material. The term "crystallite" as used herein is
inclusive of single crystals, multiple crystals, and combinations of the
latter two along with amorphous material. The crystallites have random
orientation (i.e. the major crystal axes are directed substantially
randomly from crystal to crystal) and in random locations (i.e. the
location of any given crystallite is non-predetermined, and will vary from
MRCP to MRCP).
The fencing cylinder surrounding the boat containing the substance to be
deposited serves to isolate the vaporized substance to the zone of
deposition and to prevent substance from condensing on surfaces other than
the desired surface, for example on the reverse side of the MRCP
substrate. Other equivalent means of preventing such unwanted deposition
will suggest themselves to those skilled in the art. The fencing cylinder
surrounding the boat containing the substance to be deposited further
serves to stabilize the vapor in a stable bubble shape form. Suitable
fencing cylinders used in standard bell jars measuring 12 inches in
diameter by 15 inches high are, for example, fencing cylinders measuring
140 mm in diameter and 70 mm or more in length, with the deposition source
advantageously located 40 mm above the bottom of the bell jar.
The deposition of the dynode layer may begin as soon as a portion of the
substance to be deposited enters the vapor phase, however under such
conditions, optimal performing substances may not be created. It is
preferable to delay deposition until the deposition substance has
stabilized, i.e., is molten and the resistive evaporator or equivalent
device supplies sufficient heat that neither melting nor fusion of the
source takes place during performing substance formation. It is further
preferable that no more than about 20 weight percent of the deposition
source vaporizes during performing substance formation, i.e., a
significant excess of deposition substance is preferably used. The purity
of the deposition substance should preferably exceed 99.9%, and is more
preferably of higher purity.
Investigation of the performing substance structure reveals that the
density ratio of the performing substance (.rho.) and the monocrystal
substance (.rho..sub.o), [.rho./.rho..sub.o ], is preferably from 0.003 to
0.02, more preferably 0.005 to 0.02, corresponding to a density, expressed
as a percentage, of from about 0.3-0.5 to about 2 percent compared to the
density of the substance prior to deposition. However, obtaining this
ratio is not enough. The MRCP's multiplication will take place only if the
average electron track length (L), which in turn is defined by the average
channel diameter between walls forming the channels, exceeds some definite
size depending on the deposition substance.
The physical principles of MRCP EM are as follows. The actuating medium of
the MRCP can be presented as a combination of chaotically situated dynodes
of arbitrary size and location. A tiny crystal of the particular
performing substance plays the role of the dynode. Then, electron
multiplication can be presented as a cascade process, where in each stage
of it Secondary Electron Emission (SEE) occurs. If the SEE coefficient of
the dynode is .sigma. and the energy of secondary electron is E, then
.sigma.=AE.sup..alpha., where A and .alpha. are emission property
constants of the substance in monocrystal state. E=VL/d, where V is
voltage per plate, L is electron track average length, and d is thickness
of plate. So, the overall gain G is given by:
##EQU1##
where .gamma.- is a mean caliber of MRCP channel and .gamma..ident.d/L.
The analogy of equation above with the corresponding formula of a
microchannel plate should be apparent to one with ordinary skill in the
art. The proposed technology creates a fast acting, coordinate-sensitive
EM similar to the MCP, but different from it by irregularly sized and
situated channels. In turn, the MRCP, with simple technology, results in
increased working surface at a significant decrease in cost.
Experimental results provide the basis for the following comparison of
results presented in Table 1 between a micro channel plate device (MCP)
and the micro random channel plate (MRCP) of the present invention. These
enhanced performance characteristics make possible many new and exciting
applications of MRCP technology.
TABLE 1
______________________________________
DESCRIPTION MCP MRCP
______________________________________
Thickness 300-2000.mu.
1-3000.mu.
Diameter of channels
10-100.mu. 1-20.mu.
Clearance between channels
2-10.mu. .about.0
Amount of channels
10.sup.6 cm.sup.-2
10.sup.7 cm.sup.-2
Caliber 40-100 0-3000
Channel angle 0-15.degree.
any
Surface flat any form
Maximum diameter .about.120 mm
practically
unlimited
Surface metallization
Ni,A1,NiCr any
Cross-section ratio areas
.about.60% .about.100%
Multiplication coefficient
.about.10.sup.4
.about.10.sup.6
Working voltage .about.1000 V
.about.1000 v
Electric field in channels
.about.10.sup.4 V/cm
.gtoreq.10.sup.4 V/cm
Resistance of channels
.about.10.sup.15 ohm
>10.sup.15 ohm
Capacity .about.200 pF
.about.50 pF
Pulse duration .about.1 ns <1 ns
Rise time <0,5 ns <0,2 ns
Amplitude resolution
.about.300% .ltoreq.100%
Coordinate resolution
10.sup.-3 cm
10.sup.-3 cm
Signals fluctuation
.about.100 ps
<100 ps
Dark noise .about.0,5 s.sup.-1 cm.sup.-2
.about.0,01 s.sup.-1 cm.sup.-2
Coefficient of secondary
.about.2 .about.2
emission at single event
Electron median energy
.about.7OeV .about.50eV
at exit
DETECTION EFFICIENCY
Electrons >0,1 keV
.about.50% .about.90%
Positive ions 1-1000 keV
5-85% .about.100%
Soft x-rays 2-50 .ANG.
.about.10% .about.100%
Diagnostic x-rays
.about.10% .about.20-100%
0,1-0,2 .ANG.
Neutrons .about.0,025 eV
-- .about.10%
particles .about.5 meV
-- .about.100%
______________________________________
EXAMPLE 1
A micro random channel plate electron multiplier device was prepared by
forming a performing substance layer consisting of chaotically distributed
dynodes of cesium iodide thus having random microchannels between the
cesium iodide dynodes. The substrate consisted of a nickel grid having
square holes measuring 100 by 100 .mu.m in size, spaced 20 .mu.m apart
from each other. The grid was 20 .mu.m thick, and stretched onto an
aluminum ring having a diameter of 30 mm.
Cesium iodide powder having a purity in excess of 99.9% and an average
grain size of 0.3 mm in the amount of 1.0 gram was placed in the fold
formed by creasing a 0.05 mm thick piece of molybdenum foil to form a
deposition boat measuring 12 mm by 50 mm. The deposition boat containing
cesium iodide was placed on a resistive heating stage located within a
standard bell jar measuring 12 inches (30.5 cm) diameter by 15 inches
(38.1 cm) high. The apparatus used is substantially that shown in FIG. 3.
The jar was evacuated to 10.sup.-2 torr and the cesium iodide heated to
300.degree. C., at which temperature it was maintained for a time period
of approximately 10 minutes to ensure that the cesium iodide, which, like
many salts is deliquescent, is thoroughly dry. The substrate is not
heated. The heating stage and deposition boat is surrounded by a glass
tube measuring 140 mm diameter by 70 mm length, and closed at its topmost
end by a removable screen or shutter. Following drying of the cesium
iodide, the pressure inside the jar was increased to 4 torr by admitting a
stream of dry nitrogen, and the temperature of the deposition boat was
increased to the point where the cesium iodide became molten. Following a
two minute stabilization period, the shutter at the top of the tube was
rotated away and the micro random channel plate substrate rotated into
position. The distance between boat and substrate was 60 mm. The substrate
was rotated at a rate of approximately 20 min.sup.-1 and deposition
proceeded for 10 minutes during which 10 mg of cesium iodide was deposited
as random crystallites forming a performing substance layer having a
thickness of 500 .mu.m. The micro random channel plate device was then
rotated away, the shutter repositioned over the top of the tube, and the
apparatus allowed to return to ambient conditions of temperature and
pressure.
The electrical characteristics of the MRCP thus prepared were measured,
with the following results:
______________________________________
Gain 10.sup.5 - 10.sup.6
Time Resolution .about.10.sup.-10 sec
Coordinate Resolution
.about.10.sup.-3 mm
Dark Noise .about.0.1 sec.sup.-1 cm.sup.-2
Operation Voltage .about.800 volts
Operation Vacuum .ltoreq.10.sup.-2 torr
______________________________________
As can be seen from the example, the MRCP may be economically fabricated
and has superior properties relative to conventional MCPs.
Turning now to FIG. 4, an alternative embodiment of the electron multiplier
of the present invention is presented. The MRCP is inherently coordinate
sensitive. One with ordinary skill in the art will recognize that there
are many methods for utilizing this coordinate sensitivity. One such
configuration is shown. Micro random channel plate 60 is suspended above Y
axis electrodes 64 and below X axis electrodes 62. Providing an electrical
potential between any one X axis electrode 62 and any one Y axis electrode
64 provides an indication of instant electrons at the point of
intersection between these selected electrodes. By this means, a matrix of
electron intensities can be determined by successively applying an
electric potential to each possible X axis/Y axis intersection. Thus, for
instance, a 1024.times.1024 sensor could be developed for sophisticated
imaging applications.
Turning now to FIG. 5 an alternative configuration of an electron
multiplier of the present invention is presented. A plurality of micro
random channel plates 70 are provided as well as a plurality of electrodes
72. The top electrode is given an electric potential of MN volts, this
second electrode 72 is given an electric potential of (M-1) N volts, etc.,
the next to last electrode is given a potential of N volts and the last
electrode 72 is given a potential of 0 volts or some other reference
voltage. An incident electron 74 to top plate 70 creates a series of
secondary emissions which are in turn incident to the second plate. These
incident electrons to the secondary plate in turn create secondary
emissions which are incident to the third plate, etc. and subsequent
plates, repeatedly forming a powerful and high gain electron
multiplication device. One with ordinary skill in the art will recognize
that this is but one of many possible configurations.
Turning now to FIG. 6, an alternative embodiment of the electron multiplier
of the present invention is presented. The performance of existing
electron multipliers can be significantly enhanced by the use of an MRCP
layer in conjunction with existing electron multiplier configurations. In
this particular design, a layer of the micro random channel plate is
deposited upon an existing micro channel plate device. The presence of the
MRCP layer serves to increase the gain of the existing micro channel
plate. Further, one with ordinary skill in the art will recognize that
other existing electron multiplier devices could be modified by coating
the plates, dynodes or electrodes with the random channel performing
substance used in the micro random channel plate electron multiplier
device. One additional such application would be the coating of the
dynodes in a photo electron multiplier with the micro random channel plate
layer to provide a much higher gain for the device allowing the device to
be fabricated in a much smaller configuration.
The MRCP devices of the present invention may find application in many
production applications. Examples of such applications include:
1. Dosimeters of radioactive pollution of air, water, soil, food or other
substances - the advantages of MRCPs for this application include
simplicity of production; small electric power consumption;
inexpensiveness; and universal sensing of neutrons, alpha, beta, and gamma
particles. Sizes of detectors scan vary from large stationary detectors to
microminiature detectors implanted in the case of electron wristwatch.
Watch mounted detectors may signal increased radioactivity or accumulated
doses of radiation for a given time interval.
2. Multiple use films for x-ray machines for custom service or for medical
diagnosis - the high sensitivity of MRCPs permits the reduction of dosage
levels by a factor of 100 times or more. Detector sizes may vary from very
big for whole body imaging for large subjects to as small as one toothroot
for small subject images. Similar technology can be used in detecting
defects of crystals used in microschemes (chips), and transistor
structures.
3. Microminiature detectors can be easily put in the human body for
monitoring radioactive substances used for diagnostic purposes. Such
detectors may be read through fiber optic cables or through electric
signals.
4. The quality of pictures produced by planar, black, and white, or color
video screens is significantly better than of conventional electron-ray
tubes. The thickness of such screens may be 10 mm or smaller, with an
energy consumption of no more than 1 w. Further, these designs would
produce lower levels of radiation due to the increased sensitivity of
MRCPs. Such video screens can be used in television sets with high clarity
and sharpness, in oscilloscope or computer displays etc. The useful area
of such screens could easily range from 2-3 cm to 100.times.100 cm and
beyond.
5. Coordinate sensitive super fast acting detectors are useful in any
sphere of science and technology where is a need for registration of X and
gamma rays, charged elemental particles practically of any kind and
energy, nuclear fission, thermal neutrons, etc. These detectors can be
used in nuclear physics and chemistry, in nuclear power plants, and in
space research etc. They can replace high cost semiconductor detectors,
microchannel plates, photo-electron multipliers, or electron-optic
transformers, with most characteristics of the above mentioned devices
improved.
6. Vacuum ultraviolet ray registration is also possible with MRCP
technology. Space mounted sensors could measure U.V. rays in the upper
atmosphere. Personal U.V. sensors could be produced for measuring the
total U.V. dosage for health reasons or even for proper tanning.
A possible application of the random channel plate fabricating methods of
the present invention would be in the field of microfine filters.
All-in-all, one of ordinary skill in the art will recognize a wide array of
possible applications.
While the best mode for carrying out the invention has been described in
detail, those familiar with the art to which this invention relates will
recognize various alternative designs and embodiments for practicing the
invention as defined by the following claims.
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